1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, a printer, or the like, using an electrostatic recording method or an electrophotographic recording method, and to a transfer roller which is used in such an image forming apparatus.
2. Description of the Related Art
Most conventional image forming apparatus which adopt an electrophotographic recording method use a contact transfer method whereby the generation of ozone, a material considered to be noxious, is very small. In particular, a roller transfer method having an excellent recording-material conveying property at a transfer portion has been adopted.
In the roller transfer method, a transfer nip is formed by pressing a transfer roller having an outer elastic rubber layer against a photosensitive drum, and a toner image on the photosensitive drum is transferred onto a recording material passing thru the nip by the function of a transfer bias voltage applied to the transfer roller.
Elastic sponge rollers having a hardness of 20-40.degree. (ASKER-C), obtained by forming a conductive sponge elastic layer, and whose resistance is adjusted to a value of 1.times.10.sup.6 -1.times.10.sup.10.OMEGA. by adding carbon, ion-conductive fillers or the like, on a core made of SUS (stainless steel), Fe, or the like, are generally used as the transfer rollers. Recently, in accordance with an increasing demand from a market for printing on a variety of recording materials, there has been developed image forming apparatuses using a transfer roller made of a conductive solid rubber and having a higher conveying property.
Since the conductive-solid-rubber transfer roller uses for its elastic layer a solid rubber having a high restoring force, the roller has a recording-material holding force at a transfer nip portion having a value higher than that of the conventional sponge-type transfer roller. Hence, this roller is less influenced, for example, by back tension in sheet feeding or conveying resistance produced by friction of a post card, a thick sheet, or the like, so that more stable conveyance of a recording material can be performed. Particularly, in image forming apparatuses of a so-called rapid transfer type in which the absence of an image at a central portion is prevented by scraping off toner particles from the surface of a photosensitive drum by rapidly driving a transfer roller with respect to the photosensitive drum and feeding a recording material at a speed higher than the speed of the photosensitive drum, the conductive-solid-rubber transfer roller has the feature that changes in the recording-material conveying speed due to changes in the printing ratio are smaller than in the sponge-type transfer roller.
However, the use of such a transfer roller having a high recording-material holding force causes the following problems.
First, an outline of a surrounding portion of a transfer roller will be described with reference to FIGS. 8A-8C. As shown in FIG. 8A, a transfer roller 5 is brought in pressure contact with a photosensitive drum 1 via a bearing 5e by means of a pressing spring 5d with a constant pressure. In order to reduce the cost of the transfer roller 5 and the size of the image forming apparatus, it is effective to reduce the diameter of the transfer roller 5. In this case, however, the core of the transfer roller 5 bends as shown in FIG. 8B due to the pressure contact against the photosensitive drum 1. As a result, the widths of transfer nips at both end portions become larger than at a central portion in the longitudinal direction (see FIG. 8C). In addition, while the side of the transfer roller 5 where a gear is present tends to leave the photosensitive drum 1 due to a reaction to rotation, the contact pressure and the width of the transfer nip tend to be larger at a side of the transfer roller 5 opposite to the gear side than at the gear side. If the width of the transfer nip differs in the longitudinal direction of the transfer roller 5 as described above, a transfer current flows more easily at end portions having a longer nip width than at a central portion having a shorter nip width, thereby causing unevenness in the transfer current in the longitudinal direction of the transfer roller 5. The surface of the photosensitive drum 1 is scraped off during charging. If unevenness in the transfer current occurs in the longitudinal direction of the transfer roller 5, the ablation of the photosensitive drum 1 is accelerated at the end portion of the transfer roller 5 opposite to the driving side where charging is stronger due to a larger amount of transfer current, thereby reducing the life of the photosensitive drum 1.
The solid-rubber-type transfer roller having excellent stability in conveyance of a recording material has greater hardness than the sponge-type roller, so that the difference in the width of the transfer nip in the longitudinal direction tends to increase in the solid-rubber-type roller. As a result, the amount of ablation of the photosensitive drum at the side opposite to the driving side tends to increase, thereby tending to cause a decrease in the life of the photosensitive drum. This problem is more pronounced in a low temperature/low humidity environment where the charging potential for the photosensitive drum is high.
In one type of conductive-solid-rubber transfer roller, a so-called electron-conductive rubber material in which conductivity is provided by dispersing in the rubber inorganic conductive fillers made of carbon, or the like is used. In another type, a rubber which is provided with conductivity by dispersing an ion-conductive material, such as surface active agent, or the like, is used, or a so-called ion-conductive rubber is used.
In order to respond to a recent demand from a market toward higher picture quality, transfer rollers having an ion-conductive rubber layer have been used. These transfer rollers have excellent uniformity in resistivity within an elastic layer and are therefore more suitable for high picture quality.
Electron-conductive transfer rollers have the feature that, when the rubber is crushed, the conductive structure is disrupted, thereby increasing the resistance of the rubber in proportion to the degree of disruption. An increase in the nip width due to the disruption of the rubber is compensated for by an increase in the resistance of the rubber. As a result, the above-described ablation of the photosensitive drum at the side of the transfer roller opposite to the driving side progresses less in electron-conductive transfer rollers than in ion-conductive transfer rollers. On the other hand, in ion-conductive transfer rollers, since the conductive structure is not disrupted even if the rubber is disrupted and deformed, and the resistance of the rubber does not change, local ablation of the photosensitive drum due to unevenness in the nip width tends to occur.